In the field of the vehicle seat belt apparatus designed to protect a vehicle occupant in a vehicle seat, there has been used in recent years a technique for, in an emergency or during unstable travel of the vehicle, holding or restraining a vehicle occupant by means of a seat belt to thereby restrict a posture change of the vehicle occupant. In such vehicle seat belt apparatus, a motor is timely driven by an electric-type pretensioner so that a belt reel is rotated to take up the seat belt and thereby protectively restrain a vehicle occupant and restrict a posture change of the vehicle occupant.
Further, in recent years, there has been proposed a Vehicle Stability Assist (VSA) system for use with a vehicle. The VSA is a vehicle-behavior stabilization control system which comprises a combination of an Anti-lock Braking System (ABS), Traction Control System (TCS) and vehicle side slip prevention system. “Vehicle side slip” is a traveling state in which a vehicle is turning beyond turning performance of its road wheels, and it is, for example, an oversteer or understeer condition occurring during turning travel of the vehicle.
When an oversteer condition occurs during turning travel of the vehicle, the VSA system calculates a target yaw rate, intended by a driver, on the basis of lateral acceleration, steering angle and vehicle velocity and then performs oversteer control for braking an outer (“outer” as viewed in the turning direction of the vehicle) front road wheel to reduce an actual yaw rate, because the actual yaw rate is greater than the target yaw rate. Further, when an understeer condition occurs due to acceleration during turning travel of the vehicle, the VSA system calculates a target yaw rate, intended by a driver, on the basis of a steering angle and vehicle velocity, and then performs understeer control for reducing engine output so as to increase an actual yaw rate and, as necessary, braking an inner front road wheel because the actual yaw rate is smaller than the target yaw rate.
Among the conventional techniques, where a vehicle seat belt apparatus and side slip etc. during turning travel of the vehicle are correlated, is a vehicle occupant restraining/protecting system disclosed in Japanese Patent Application Laid-Open Publication No. 2001-122081. When side slip or the like of a vehicle body has occurred during turning travel of a vehicle, the vehicle occupant restraining/protecting system disclosed in the 2001-122081 publication activates an electric-type pretensioner, which takes up a seat belt using yaw rate information acquired via a yaw rate sensor, yaw rate information acquired via a yaw rate sensor and the like, to pull the seat belt and thereby realizes effective protective restraint of a vehicle occupant.
However, when side slip of the vehicle body has occurred, the restraining/protecting system disclosed in the 2001-122081 publication merely detects the side slip and pulls the seat belt through activation of the electric-type pretensioner of the vehicle seat belt apparatus. Therefore, depending on the circumstances, a vehicle occupant restrained by the seat belt may have an unpleasant, uncomfortable feeling with respect to the operation of the vehicle seat belt apparatus.
When some abnormal condition, such as an oversteer or understeer condition, occurs during turning travel of a vehicle, and if the vehicle is equipped with a VSA system, behavior of the vehicle is stabilized on the basis of control by the VSA system. Thus, it is desirable that an electric-type pretensioner in the vehicle seat belt apparatus be triggered by the VSA system and effect optimal vehicle occupant restraint, in association with the vehicle-behavior stabilization control operation of the VSA system, in such a manner that any unpleasant, uncomfortable feeling is not imparted to the vehicle occupant and steering operation by the vehicle occupant (if the vehicle occupant is a driver of the vehicle) is not disturbed due to the restraining force of the belt.
Further, when the driver has made a sharp turn by rapid steering operation of the steering wheel, such rapid steering operation is determined on the basis of a signal generated by a steering angle sensor added to a steering device. In such a case too, it is desirable that the electric-type pretensioner in the vehicle seat belt apparatus be arranged to effect optimal vehicle occupant restraint by being triggered by the signal indicative of the rapid steering operation, as in the case where abnormality of vehicle behavior is dealt with by the VSA system.